Icelandic police said Thursday they had arrested four people in a large-scale operation linked to preparations of a "terrorist attack".

Speaking at a press conference, police said several semi-automatic weapons, including 3D-printed ones, had been seized at nine different locations, as well as "thousands" of rounds of ammunition.

"The origin of the police operations yesterday... was in the context of an investigation into the preparation of a terrorist attack," Karl Steinar Valsson, National Commissioner of the Icelandic Police, told reporters.

Four Icelanders in their 20s were arrested in Kopavogur, a suburb of the capital Reykjavik, and the southwestern town of Mosfellsbaer in a large-scale operation on Wednesday involving some 50 men, police said.

Two of them had been remanded in custody on Thursday.

While potential motives were unknown, the targets are believed to be "various institutions of society" and "citizens of the state", according to Valsson, possibly including the parliament and the police itself.

Police also said they were looking into where there were links between the men and extremist organizations and that they were in contact with foreign authorities.

The nation of 375,000 people has topped the Global Peace Index since it was included in the ranking in 2008 and considered the "most peaceful country in the world".

Violence is rare, evidenced by one of the lowest crime rates in the world, although a surge in violent crime in recent years has worried the authorities.